1

Engineering Graphics

SCREW THREADS, BOLTS and NUTS

Learning Objective 1 Identify and use of different grades of pencils and other drafting instruments hich are used in engineering field

2 Draw free hand sketches of various kinds of objects.

3 Utilize various types of lines used in engineering drawing.

4 Read and apply different dimensioning methods on drawing of objects.

2

3

>Thread is the helical grooves which are opened to inner and outer surfaces.

Fig. 1 shows the screw thread terminology.

>External thread (screw/vida - bolt/cıvata - stud/saplama): A thread on

the external surface of a cylinder.

>Internal thread (nut/somun): A thread on the internal surface of a cylinder.

>Major diameter (diş üstü çap): The largest diameter of a screw thread.

>Minor diameter (diş dibi çap): The smallest diameter of a screw thread.

>Pitch diameter (bölüm çapı): The diameter of an imaginary cylinder, the

surface of which cuts the thread forms where the width of the

thread and groove are equal. Figure 1 1

Threaded Fasteners

4

> Crest: The edge or surface that joins the sides of a thread and is farthest

from the cylinder or cone from which the thread projects.

> Root: The edge or surface that joins the sides of adjacent thread forms and

coincides with the cylinder or cone from which the thread projects.

> Depth of thread: The distance between crest and root measured normal to

the axis.

> Pitch (hatve, adım): The distance between corresponding points on

adjacent thread forms measured parallel to the axis.

> Right-hand thread: A thread that when viewed axially winds in a clockwise

and receding direction. Threads are RH unless otherwise specified.

> Left-hand thread: A thread that when viewed axially winds in a

counterclockwise and receding

direction. All left-hand threads are designated LH. Figure 1 2

Threaded Fasteners

5

> Lead: The distance a threaded part moves axially with respect to a fixed mating part, in one complete revolution.

> Single thread: A thread having the thread form produced on only one helix of cylinder. On a single thread, the lead and pitch are

equivalent. Threads are always single unless otherwise specified (Fig. 2).

> Multiple thread: A thread combination having the same form produced on two or more helices. For a multiple thread, the lead is

an integral multiple of the pitch (e.g. on a double thread, lead is twice the pitch). A multiple thread permits a more rapid advance

without a coarser (larger) thread form (Fig. 2).

Threaded Fasteners

6

Figure 2 3

7

> Profiles of standard screw thread forms used in industry are shown in Fig. 3.

Screw Thread Profiles

8

Figure 3

4

9

> External (Fig. 4) and internal (Fig. 5) screw threads are illustrated below.

> Thread notes are used in drawings in order to define type and size of screw threads.

British Type

.250-20 UNC-2A-LH

a b c d e

a: Major diameter

(inch) b: Threads per

inch

c: Form (i.e. Unified National Coarse) d:

External thread (B for internal)

e: Left-hand thread (RH for right-hand)

Figure 4

Screw Thread Representation

y: Major diameter (mm)

z: Pitch (mm) x y z

Metric Type

x: Metric screw thread

M20 x 2

10

Figure 5

5

11

> Screw threads for blind holes with partially tapped (Fig. 6) and bottom tapped

(Fig. 7) are shown below.

> The chamfered view of screw threads is drawn only for large chamfers (Fig.

8).

> There are also threaded parts on pipes for special applications (Fig. 9).

Figure 8

Figure 6

Threads for Specific Purposes

12

Figure 7 Figure 9

6

13

Figure 11

> Fig. 10 shows a nut (unfaced and faced at bottom side) and a bolt having hexagonal

heads.

> There are five commonly used type of fasteners in industry using bolts, nuts, cap screws

and studs (Fig. 11).

> In most cases, the combination of bolt and nut, stud or cap screw with hexagonal heads

are used for holding parts together.

Bolts, Nuts and Studs

Nut

Bolt Figure 10

14

7

15

Assembly Drawings of Bolted and Stud Joints

s

h

m

n

H

c

l0

l

m

n

H

s

c

l1

l3

l2

l0

l

16

8

3 1

m = tabulated

n = tabulated

l = l + 0.4d

2 1 l = l + 0.5d

1 l = 1.25d

0 l = 2d to 2.5d

R = 0.5c 2

K = 3d

l = tabulated

w

A = 1.1d

s = 0.15d

c = s to 2s

R = 1.5d

r = based on R

Bolted Joint

Figure 12

D

Stud Joint

Figure 13

D

D

w

K

D

w

K

17

Figure 14

Figure 15

Various Types of Bolts and Nuts

18

9

Questions:-

Q.1Draw Assembly of nut and bolt.

Q.2 Define nut,bolt and their types.

Q3 Draw front and top view of hexagonal nut, bolt.